Abradable protective coating for compressor casings



Nov. 28, 1961 v, K. EDER 3,010,843

ABRADABLE PROTECTIVE COATING FOR COMPRESSOR CASINGS Filed April 28, 1958.4 f ,7 f [/9 I v 47 A; LA?

United States Patent .Ofi

3,016,843 Patented Nov. 28, 1961 ice This invention relates toimprovements in compressors and more particularly to improvements in theefficiency of axial-flow compressors by application of a suitable resinimpregnated metal coating to the inner Wall surface of thecompressorhousing.

A simplified flow diagram of the method is as follows:

Clean part Spray porous tnetal coating to desired thickness on clIe anedsurface of part maintained at 150 to 200 Impregnate coating withthermosctting resin Typical axial-flow compressors, such as those usedin modern turbine engines, include a rotor which Carries I havediscovered that the clearance between the blade tips and the compressorhousing in an axial-flow compressor can be effectively reduced to aminimum by providing the interior of the compressor housing with acoating of a relatively soft metal which is adapted to be cut away bythe tips of the moving blades to provide a minimum clearance for optimumoperating efiiciency, the method of application of said material to saidhousing being to apply a machinable porous metal coating and a resinousplastic coating to fill the pores to produce a tightly adherent,non-porous, substantially warp and crack resistant coating which isreadily machinable and protects the housing from corrosion.

The invention will be best understood in connection with theaccompanying drawing, in which:

FIG. 1 is a fragmentary view, partially in section, of a multistageaxial-flow air compressor incorporating the invention; and

FIG. 2 is a fragmentary view, partially in section, of one stage ofanair compressor taken on line 2-2 of FIG. 1.

Referring now to the drawing in detail, a turbo-compressor indicatedgenerally at 1, is shown in a housing 3, only so much of the compressorbeing shown as is necessary to illustrate the invention. The rotor isfabricated to carry a plurality of rows of rotor blades 5, the bladesbeing supported on the rotor shaft in any manner well rows of outwardlyextending rotor blades or vanes and a compressor housing which carriescorresponding rows of stator vanes. The body of the rotor may be formedof a suitable high strength forgeable material such as an alloy oftitanium, aluminum, and/ or steel while the rotor vanes, generallydovetailed into the rotor, may be of stainless steel or the like. g j

The compressor housing or casing, which may be cast from stainlesssteel, aluminum, magnesium, or the like, and various alloys thereof,generally is a two-piece assembly split on a plane through thecompressor axis. Secured to the inner circumference of the housing arelongitudinally spaced rows of stator vanes of stainless steel or thelike which project inwardly between thecorresponding rows of rotorvanes. I

Up to the present time, one of the primary difficulties with axial-flowcompressors has been the excessiveenergy loss due to the leakage of airpast the rotor blade tips.

The amount of air leakage depends to a large degree upon the clearancebetween the tips of the rotor blades and the compressor housing. Thisclearance in turn depends on the rigidity and dimensional stability ofthe compressor. In addition to the warpage and elastic deformationencountered in operation, the differential expansion of the compressorparts over the wide range of temperatures encountered in use, makes ithighly impractical to known in the art and not shown. Extending inwardlyfrom the housing 3 between the rows of rotor blades 5 are the rows ofstator vanes 7. The stator vane rows are supported byrings 9 which I aresuitably secured in annularvgrooves 11 in the housing3.

, The present invention is directed to methods for reducing theclearance between the tips 13 of the rotor blades 5 and the innercircumferential surface 15 of the compressor housing. In accordance withthis invention the portions of the housing adjacent the paths describedby the tips 13 manufacture a compressor having a minimum clearance foroptimum efliciency. Not only would more costly finf ishing andinspection operations be required in manufacture, but in many cases thedimensional instability of the closely fitting parts would result indamage to the com pressor by scoring or gouging of thehousing orbreakage v of the rotor vanes.

Accordingly, the principal object of this invention is to provideeffective methods for minimizing the clearance between the rotor bladetips and the compressor housing and for minimizing corrosion of thehousing. A further object is to provide a method for applying a readilymachinable heauresistant coating to the interior of the com pressorhousing to reduce the clearance between the blade tips and the housing.Other objects and advantages will more fully appear from the descriptionwhich follows.

of the moving blades 5 are provided with a porous coating 17 of arelatively soft metal such as aluminum, bronze or zinc which is adaptedto be readily machined by the blade tips to establish a minimumoperating clearance between the blade tips and the rotor housing. Asindicated in the fragmentary end view in FIG. 2, the coating 17 extendsaround the entire circumferential portion of the housing I adjacent themoving rotor blade tips. Also, as is described more fully hereinafter,the surface of the housing 3 which is coated is roughened as by cuttingshallow threads 19 therein as shown in FIG. 1.

I have found that by impregnating the porous coating 17 with aheat-resistant resinous plastic not only are the pores sealed thusprecluding corrosion of the housing, but that themachinability of theresultant coating is very greatly. improved as is also the surfacesmoothness and uniformity. Extended development and tests demonstratedthat a metal coating having the desired properties noted above and whichis adherent to the base metal and possesses a uniform surface free fromblisters, chips or other objectionable defects may be obtained by thehereinafter described methods. I

It is essential that the surface to be coated be properly cleaned andconditioned prior to application of the metal coating. Thesurface shouldbe thoroughly cleaned to remove dirt, grit, and oils by solvent cleaningas by vapor degreasing in trichloroethylene or by dip-washing in apetroleum solvent. The surface is then conditioned to enable a tightlyadherent coating by machining a shallow thread in the surface and/orgritblasting or shotting to provide a roughened surface. The resultantaluminum,

magnesium or steel surface should have a rather fine texture' and shouldbe substantially uniform in quality.

The porous metalcoating is then applied to the desired a metallizingspray gun. I have found it necessary topreheat the surface to be coatedto a temperature of from about 175-200 F. During the spraying operationI have found it necessary to maintain the temperature of the partbetween 150-200 F. Parts deviating from these temperatures are apt tohave an inferior bond between the coating metal and the base metal. Thedesired temperature control may be conveniently achieved by applying ahot air blast to the opposite side of the part being treated.

In applying aporous bronze coating to a magnesium compressor housingsurface, I found that a coating thickness of from 0.003 to 0.005 inchWithout any intermediate bonding layer was sufficient to produce thedesired characteristics. In applying a porous aluminum coating to asteel compressor housing surface it was found that an intermediatebonding layer of stainless steel produced the best results. My preferredmethod is to spray coat a layer of stainless steel of from about 0.002to 0.005 inch thickness followed by the application of a porous aluminumcoat of from about 0.030 to 0.035 inch thickness. It should be notedthat other intermediate layer materials may be utilized, i.e., bronze,zinc, copper.

While the porous metal coating is machinable, I have found that theimpregnation thereof greatly improves the machinability While at thesame time producing a uniformly smooth surface and precluding corrosionof the base metal. impregnation is accomplished by using a hightemperature resistant thermosetting resin such as any of the well knownepoxy or silicone resins in sufficient solvent or thinner to produce asolution of the desired viscosity. About equal parts by volume of epoxyresin and thinner has been found to be suitable. Similarly, a mixture ofabout 18.022.0 parts by weight silicone resin solids and about 78.0-82.0parts solvent is satisfactory. I have found that a minimum of two coatsof impregnant should be applied in any suitable manner, as by brushing,spraying, dipping or rolling. Complete impregnation is accomplished whenresin is still on the surface after about 20 minutes of air drying afterapplication, this dry period being used after each coat.

After complete impregnation, the part is subjected to baking at elevatedtemperatures in progressive steps in order to thoroughly dry and curethe materials. I have found it to be satisfactory if the impregnatedcoating is first baked at about 150 F. for at least one-half hourfollowed by a bake at about 450 F. for one hour. Where the porous metalcoating is relatively thick, as in the case of aluminum, a final bake isdesirable at about 625 F.

for a period of about 3 hours. Upon cooling to room temperature, sandingwith steel wool or other finishing may be used to obtain a smootherfinish, if desired.

While I have described my invention in terms of applying the coatingdirectly to the housing of the compressor, it should be understood thatit may be applied to parts positioned on the housing. Other embodimentsmay be apparent to those skilled in the art and such embodiments arewithin the scope of my inventionas defined by the claims which follow. 1

I claim:

1. The method for forming a machinable metal coating on a metalpartadapted to form that portion of an axial flow compressor housingadjacent the path of the rotor blade tips comprising the steps ofcleaning the surface of the part to be coated, spraying'a porous metalcoating on said surface to the desired thickness, controlling thetemperature of said surface during coating to maintain atemperature ofbetween 150 and 200 F. and thus preclude inferior bonding between saidcoating and said surface, and impregnating said coating with a hightemperature thermosetting resin to fill the pores thereof.

2. A method as set forth in claim 1 including the steps of tougheningthe surface to be coated and preheating the part to a temperature ofabout 175200 F. prior to coating.

3. A method as set forth in claim 2 including the steps of subjectingthe coated and impregnated part to baking at successive elevatedtemperatures not to exceed about 625 F.

4. A method for forming a machinablepo'rous bronze coating on thatportion of a magnesium housing of an axial flow compressor adjacent thepath of the rotor blade tips comprising the steps of cleaning thesurface to be coated, roughening said surface to promote formation of atightly adherent coating, preheating said surface to a temperature ofabout 175-200 F., spraying a porous bronze coating on the surface to thedesired thickness, controlling the temperature of said surface duringcoating to maintain a temperature of between 150 and 200 F. to thuspreclude inferior bonding between said coating and said surface, andimpregnating said porous coating with a high temperature thermosettingresin.

5. A method as set forth in claim 4 including the steps of baking theimpregnated coating at a temperature of about 150 F. for a period of atleast /2 hour followed by baking at a temperature of about 450 F. for aperiod of about one hour.

6. A method as set forth in claim 5 wherein the thickness of saidcoating is from about 0.003 to 0.005 inch and said impregnating resin isan epoxy resin.

7. A method for forming a machinable porous aluminum coating on thatportion of a steel housing of an axial flow compressor adjacent thepathof the rotor blade tips comprising the steps of cleaning the surface tobe coated,

roughening said surface, preheating said surface to a temperature ofabout 175-200 F., spraying an intermediate bonding layer of stainlesssteel, spraying a porous aluminum layer on said bonding layer to thedesired thickness, controlling the temperature of said surface duringcoating to maintain a temperature of between 150 and 200 F. to thuspreclude inferior bonding between said coating and said surface, andimpregnating said coating with a high temperature thermosetting plastic.

8. A method as set forth in claim 7 including the steps of baking theimpregnated coating at a temperature of about 150 F. for a period of atleast /2 hour followed by baking at a temperature of about 450 F. for aperiod of about one hour and at a temperature of about 625 F. for aperiod of about three hours.

9. Amethod as set forth in claim 8 wherein the thickness of the bondinglayer is from about 0.002 to 0.005 inch, the thickness of the aluminumis from about 0.030 to 0.035 inch, and the impregnating resin a siliconeresin.

4. A METHOD FOR FORMING A MACHINABLE POROUS BRONZE COATING ON THATPORTION OF A MAGNESIUM HOUSING OF AN AXIAL FLOW COMPRESSOR ADJACENT THEPATH OF THE ROTOR BLADE TIPS COMPRISING THE STEPS OF CLEANING THESURFACE TO BE COATED, ROUGHENING SAID SURFACE TO PROMOTE FORMATION OF ATIGHTLY ADHERENT COATING, PREHEATING SAID SURFACE TO A TEMPERATURE OFABOUT 175*-200*F., SPRAYING A POROUS BRONZE COATING ON THE SURFACE TOTHE DESIRED THICKNESS, CONTROLLING THE TEMPERATURE OF SAID SURFACEDURING COATING TO MAINTAIN A TEMPERATURE OF BETWEEN 150* AND 200*F. TOTHUS PRECLUDE INFERIOR BONDING BETWEEN SAID COATING AND SAID SURFACE,AND IMPREGNATING SAID POROUS COATING WITH A HIGH TEMPERATURETHERMOSETTING RESIN.